Cochlear implants often operate in a monopolar stimulation configuration, with current being sent out on an intracochlear electrode and returned via a distant extra-cochlear ground electrode. A monopolar stimulation configuration produces a relatively wide excitation pattern, does not allow for control of current flow within the cochlea, and often produces unwanted percepts. Multipolar stimulation configurations (e.g., partial tripolar stimulation configurations where current is sent out via an intracochlear electrode, a portion of the current is returned via two flanking intracochlear electrodes, and a remaining portion of the current is returned via the extra-cochlear ground electrode) may focus the electric field produced by current applied to an electrode (i.e., minimize current spread within the cochlea) and thereby result in better control of current flow, reduce unwanted percepts, and improve speech perception.
Unfortunately, it has been discovered that, in some patients, multipolar stimulation configurations actually result in broader, not narrower, excitation fields compared to monopolar stimulation configurations. This is likely due to the fact that the amount of current required to achieve a most comfortable stimulation level (“M level”) associated with an electrode increases exponentially with the amount of focusing used with respect to the electrode. Hence, if a particular electrode within a patient is not positioned correctly, or if neural survival within a region associated with the electrode requires a relatively high current to achieve an M level associated with the electrode, a monopolar stimulation configuration may actually result in a narrower electric field than a multipolar stimulation configuration.